When a vehicle travels on a cant road with an inclination in the width direction of the road (toward a road shoulder) or when the vehicle travels in side winds, the vehicle pulls to the lower side in the inclination direction because of the gravity on the cant road, or the vehicle pulls to the leeward in the side winds because of the wind power although the vehicle intends to travel straight ahead. That is, one-side pull of the vehicle is generated. Hence, to travel straight ahead against the one-side pull of the vehicle, a driver has to rotate a steering wheel to the upper side in the inclination direction or the windward. That is, even during the straight-ahead travel, the driver has to continuously give a steering force in a direction opposite to a direction to which the vehicle pulls (the one-side pull direction).
For example, Japanese Unexamined Patent Application Publication No. 2001-1923 discloses a vehicle steering device that, in the case that a disturbance, which affects a vehicle behavior, is generated (for example, during travel on the cant road or during travel in the side winds), can obtain a disturbance effect value D meeting the degree of effect of the disturbance with respect to the vehicle behavior, and can control a steering assist force Ta in accordance with the disturbance effect value D. In Japanese Unexamined Patent Application Publication No. 2001-1923, a dead band is provided around the midpoint of a steering torque, and the steering assist force is generated if the steering torque exceeds the dead band, to cancel the disturbance (the inclination of the cant road or side winds).
However, in Unexamined Patent Application Publication No. 2001-1923, since the control is executed with the dead band provided for the steering torque as shown in FIG. 4 etc. thereof, the driver receives a load until the steering torque exceeds the dead band, and thus comfortableness may be decreased. Also, in the case of a driver who is unskilled in driving, the driver may excessively rotate the steering wheel for a turn. Owing to this, when the turn is ended, the vehicle may be steered by a certain degree in a direction opposite to the turn direction. At this time, the steering in the opposite direction is considered as the result of the effect by the cant road surface, and control may not be properly made for the one-side pull.